The present invention relates to an edge type surface light source to be used as a backlight for a transmission liquid crystal display, an advertising board or the like, and a light guide plate and a lens film to be used as the component parts of the surface light source. More particularly, the present invention relates to a surface light source capable of emitting light in a narrow directional luminance distribution, at a low loss in quantity of light and in a uniform surface luminance distribution, and consisting of a relatively small number of parts, and to a light guide plate and a lens film suitable for use as the components of the surface light source assembly.
Backlights for transmission displays, such as liquid crystal displays, are classified by the position of their light sources into side backlights provided with a light source beside a side edge thereof, and direct backlights provided with a light source behind them. Most liquid crystal displays employ a side backlight, because a liquid crystal display provided with a side backlight needs a small space and can be formed in a small thickness.
Referring to FIG. 10 showing a conventional side surface light source assembly by way of example in a sectional view, a light guide plate 101 receives light through a side surface thereof serving as a light entrance surface 101a, and emits light through a two-dimensional light exit surface 101b. A back surface 101c lies opposite to the light exit surface 101b. The back surface 101c is inclined to the light exit surface 101b so that the thickness of the surface light source assembly decreases linearly with distance from the light entrance surface 101a. A linear light source 102 is disposed near and in parallel to the light entrance surface 101a. A reflector 103, such as a light reflecting film or the like, is disposed near and below the back surface 101c. A light diffusing film 104 is disposed near and over the light exit surface 101b, and a lens film 105 is disposed near and over the light diffusing film 104.
As shown in FIG. 10, this light guide plate 101 is formed by forming a diffusive reflecting layer 110 on the back surface of a plate of a predetermined shape produced by injection molding or by cutting a plate of a predetermined shape out of a transparent resin block, such as an acrylic resin block, and polishing the surfaces of the plate.
The linear light source 102 is a cold-cathode tube. The reflector 103 is a resin film coated with a metal thin film deposited by evaporation. The light diffusing film 104 is an opal resin film. The lens film 105 is a resin film provided with a plurality of triangular prisms. The diffusive reflecting layer 110 is a layer of numerous minute white dots formed on the back surface of the light guide plate 101 by printing. The density of the minute white dots is increased with distance from the light entrance surface 101a near the light source 102 to compensate the reduction of the quantity of light with distance from the light entrance surface 101a so that the light guide plate 101 has a uniform surface luminance distribution. The light diffusing film 104 diffuses the light leaving the light exit surface 101b of the light guide plate 101 so that the white dots forming the diffusive reflecting layer 110 and the light source 102 are not directly visible from the side of the light exit surface 101b.
This conventional side surface light source assembly has the following drawbacks. When forming the diffusive reflecting layer 110 by forming the minute white dots by a screen printing process on the back surface of the light guide plate 101, the dots are not formed in a desired shape and a desired area if the meshes of a printing screen are clogged with the ink, which reduces the yield of a guide plate forming process. The light diffusing film 104, i.e., an additional part, is necessary to make the printed dots invisible. Since the surface light source assembly needs the printed dots and the light diffusing film 104, the surface light source assembly manufacturing process needs additional work for printing the dots and combining the light diffusing film 104 with the light guide plate 101, which unavoidably increases the manufacturing cost of the side surface light source assembly.
A surface light source assembly employed in some display is required only to emit light in a narrow directional luminance distribution around a specific direction, such as a direction normal to its light exit surface. If the conventional surface light source assembly is employed in such a display required to be sufficiently bright in a narrow angle of view, light is diffused in an excessively wide range and light is used inefficiently.